The present invention relates to the design of two-ply pneumatic radial ply runflat passenger tires.
Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without simultaneously compromising vehicle handling over a distance from the place where the tire has lost its pressure to a place desired by the driver, such as a service station, where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by an object such as a nail or other sharp object.
Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated, or what would generally be called xe2x80x9cflat,xe2x80x9d condition. The conventional pneumatic tire collapses upon itself when it is uninflated and is carrying the weight of a vehicle. The tire""s sidewalls buckle outward in the portion of the tire where the tread contacts the ground, making the tire xe2x80x9cflat,xe2x80x9d at least in that ground-contacting region.
The term xe2x80x9crunflatxe2x80x9d is generally used to describe a tire that is designed such that the tire structure alone, in particular the sidewalls, has sufficient rigidity and strength to support the vehicle load when the tire is operated without being inflated. The sidewalls and internal surfaces of the tire do not collapse or buckle onto themselves, and the tire does not otherwise contain or use other supporting structures or devices to prevent the tire from collapsing.
An early example of a runflat tire design is described in U.S. Pat. No. 4,111,249, entitled the xe2x80x9cBanded Tire,xe2x80x9d in which a hoop or annular band approximately as wide as the tread is circumferentially deployed beneath the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition.
Another approach taken has been simply to strengthen the sidewalls by increasing their cross-sectional thickness so as to resist the bending that takes place in the portions of the sidewalls that are adjacent to the ground-contacting portion of the tread. However, due to the large amounts of rubber required to stiffen the sidewall members, flexure heating becomes a major factor in tire failure during runflat operation, especially when the uninflated tire is operated at high speeds. Pirelli discloses such a runflat tire design in European Pat. Pub. No. 0-475-258A1.
In general, runflat tires incorporate sidewalls that are thicker and/or stiffer than typical non-runflat tires so that the tire""s load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The typical methods used in sidewall thickening and stiffening include the incorporation of circumferentially disposed wedge inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, each sidewall is thickened in such a way that its overall thickness, in the region between the bead and the tread shoulder, is on the order of the thickness of the tread regions of the tire.
The reinforced sidewalls of such tires, when operated in the uninflated condition, experience a net compressive load in the region of the sidewall closest to the road-contacting portion of the tread. Also, the bending stresses on the reinforced sidewalls are such that, during runflat operation, the outer portions of the sidewalls experience tensile forces while the inner portions experience compression stresses.
A Goodyear patent U.S. Pat. No. 5,368,082, by Oare et al (""082), disclosed a low aspect runflat, pneumatic radial ply tire which employs special sidewall inserts to improve stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty that could be offset by the elimination of a spare tire and the tire jack. However, this weight penalty becomes even more problematic in the design of tires having higher aspect ratios.
Two U.S. Pat. Nos. 5,427,166 and 5,511,599 of Walter L. Willard, Jr., show Michelin tires that disclose the addition of a third ply and the addition of a third insert in the sidewall to further increase the runflat performance of the tire over that of the ""082 patent. These two later patents discuss some of the load relations that occur in the uninflated condition of the tire and they demonstrate that the concept set out in the ""082 patent can be applied to additional numbers of plies and inserts.
In general, runflat tire design is predicated upon the use of reinforcing wedge inserts inside each sidewall flex area. The wedge inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the absence of air pressure during runflat operation. The ""082 patent teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or innermost ply.
While the high resistance to compression deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, the use of multiple plies and, in each sidewall, more than one reinforcing wedge insert has drawbacks which include the above mentioned increase in tire weight and cyclical-flexure-induced heating. Such designs also increase the tire""s complexity in ways that adversely affect manufacturing and quality control.
Clearly, the goal in runflat tire design is to provide a low-cost, light-weight tire that gives good runflat vehicle handling as well as good service life during runflat operation.
It is an object of the present invention to provide a runflat radial tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
One object of the present invention is to provide a runflat tire having a tread that resists upward buckling during runflat operation.
Another object of the present invention is to provide a runflat tire that is of light-weight construction.
Still another object of the present invention is to provide a runflat radial tire having sidewalls that resist deformation in the region of ground contact during uninflated operation.
Yet another object of the present invention is to provide a runflat radial tire having a ply structure in which the outermost ply is reinforced with a light-weight, high-modulus material, specifically aramid.
Another object of the present invention is to have the aramid-reinforced, outermost ply clamped to the bead structures.
A further object of the present invention is to prestress in tension, during the manufacturing process, the aramid-reinforced outermost ply.
A yet further object of the present invention is to provide a runflat tire having good full-inflated handling characteristics as well as good runflat handling properties and runflat operational life.
Still another object of the present invention is to apply the inventive concept to a variety of alternative tire designs.
The present invention relates to a pneumatic radial ply runflat tire having a tread, a carcass comprising a radial ply structure, a belt structure located between the tread and the radial ply structure, two inextensible beads, and two sidewalls, each of which is reinforced by wedge inserts. The ply structure comprises an inner ply and an outer ply. The outer ply is reinforced with aramid, a material having low density and high modulus of elasticity. The outer ply is clamped around the beads and prestressed in tension during manufacture of the tire.